Power press die cushion with air decelerator

ABSTRACT

A pneumatic die cushion for a power press. The cushion includes an air chamber and a separate cavity which respectively expand and contract in volume as the die cushion returns its original height following a pressing operation. A decelerator valve, which is responsive to the air pressures within the chamber and cavity, controls the rate at which air is bled from the cavity to gradually reduce the speed at which the cushion returns to its original height.

United States Patent 1 [111 3,923,294

Gold et al. Dec. 2, 1975 1 1 POWER PRESS DIE CUSHION WITH AIR FOREIGN PATENTS OR APPLICATIONS DECELERATOR 1.017.655 1/1966 United Kingdom 267/119 [75] Inventors: Robert M. Gold, Minneapolis.

Robert Gold Primary E.\'aminerJames B. Marbert Phoemx Attorney, Agent. or FirmJames R. Haller; H. Dale [73] Assignee: Dayton Rogers Manufacturing Co., Palmatier Minneapolis, Minn. 22 Filed: 0a. 21, 1974 1571 ABSTRACT [21] APPL No: 516,751 pneumatic die cushion for a power press. The cushion includes an air chamber and a separate cavity which respectively expand and contract in volume as [52] U.S. C1. 267/119 th di u hion returns its original height following a [51] Int. CLZ F16F 5/00 pressing operation A decelerator valve, which is re [58] Field Of Search 267/119 sponsive to the air pressures within the chamber and cavity, controls the rate at which air is bled from the 1 1 References Cited cavity to gradually reduce the speed at which the UNITED STATES PATENTS cushion returns to its original height.

3,013,791 12/1961 Gold (it al. 267/119 11 Claims 4 Drawing Figures 3,596,896 8/1971 Kraft 267/119 -/a.s' 4 I =1 I: /86

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US. atent Dec. 2, 1975 Sheet 2 of2 3,923,294

POWER PRESS DIE CUSHION WITH AIR DECELERATOR BACKGROUND OF THE INVENTION To avoid damaging expensive power press equipment, power press dies are often mounted so as to yield or give resiliently downwardly against pneumatic die cushions when a die is struck by a ram, the cushion at least partially absorbing the impact of the ram upon the die. A die cushion is described in commonly owned U.S. Pat. No. 3,013,791.

As a die cushion is returned upwardly to its original height following a forming operation, the cushion may come into contact with upper stops of the press with a jolt. With the advent of high speed presses, the rapid return travel of the die cushion ending in a jolting stop has given rise to a hammering action due to the large moving mass of the die striking against the upper stops or bolster plate of a press. The hammering action not only produces excessive noise, but may lead to damage of power presses and related equipment.

An apparatus for avoiding the hammering which occurs with high speed power presses employing pneumatic die cushions is much to be desired and would greatly advance the art.

BRIEF DESCRIPTION OF THE INVENTION The invention relates to power presses, and more particularly to a pneumatic die cushion which is depressed under the force of a ram and which returns to its original height at a controlled decreasing speed following a pressing operation, the cushion coming gently to rest at its original height. The cushion includes an air chamber under substantially constant pressure and which respectively contracts and expands in volume as the cushion is first depressed under the downward force of a power press ram and as the cushion returns to its original height when the ram force is removed. The cushion also includes a cavity which respectively expands and contracts in volume as the height of the cushion decreases and increases. A decelerator valve is provided to bleed air in a controlled manner from the cavity to control the speed of return of the cushion to its original height. The valve includes means responsive to the air chamber and cavity pressures to open the valve when a given ratio of cavity pressure to air chamber pressure has been exceeded. The cavity and air chamber each have effective hydraulic areas over which their pressures are applied in opposition to each other, and the decelerator valve desirably includes a plug and valve seat receiving the plug. The plug on one side of the valve seat has an effective hydraulic area exposed to the air pressure of the air chamber, and on the other side of the valve seat the plug has an effective hydraulic area exposed to the cavity pressure. The ratio of the latter plug area to the former plug area is chosen to be greater than the ratio of the effective hydraulic area of the cavity to that of the air chamber, whereby the valve will open and bleed air from the cavity before the pressure in the cavity increases to balance the force exerted by the air chamber. The valve may include an auxilliary bypass channel communicating the cavity with the atmosphere, the channel including a bleeder valve with an exteriorly accessible setting stem for regulating the flow rate of air through the channel, thereby facilitating fine adjustment of the speed of return of the die cushion to its original height.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1, 2 and 3 are broken away, semi-schematic, cross-sectional views of a pneumatic die cushion of the invention, FIGS. 1 and 2 showing the cushion at its original height and its depressed height, respectively, and FIG. 3 showing the cushion at an intermediate position as it returns to its original height; and

FIG. 4 is a cross-section, broken away view of a decelerator valve employed in the die cushion of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIGS. 13, a pneumatic die cushion of the invention is designated generally as 10. The cushion includes an upright cylinder 14 having an upper strike plate closure 14.1 which is struck down wardly by a press element 12 as the ram ofa press (not shown) moves downwardly in a pressing operation. The cylinder 14 includes upright, cylindrical sidewalls 14.2. A rigid piston 16, which is adapted for rigid mounting to a base or other support, is provided with an upright, centrally positioned piston rod 16.1 having a transverse head or plate 16.2 at its upper end, the periphery 16.3 of the plate slideably engaging the inner surfaces 14.3 of the upright side walls of the cylinder. A lubricated packing gland 16.4 may be provided in a peripheral groove of the plate 16.2 to render the sliding contact between the plate 16.2 and the cylinder side walls 14.3 substantially airtight. The upper surface 16.5 of the piston plate 16.2 is spaced beneath the lower surface 14.4 of the strike plate 14.1 when the cushion is at its original height as shown in FIG. 1, the thus-described spaced surfaces and the walls of the cylinder defining an air chamber 21, as will be more fully explained below.

The upright cylindrical walls 14.2 of the cylinder are provided, at their lower ends, with wall extensions 18 which may be welded or otherwise afixed to the walls of the cylinder and which may be spaced apart the same distance as are the cylinder walls. The piston 16 is provided with a stationary bottom plate 17 having cylindrical, outwardly spaced upright side walls 16.6, the latter side walls having outer surfaces 16.7 which slideably engage the inner surfaces 18.1 of the wall extensions 18. The upper periphery of the wall 16.6 may be provided with a peripheral groove carrying a lubricated packing gland 16.8 to render the seal between the walls 18 and 16.6 substantially airtight.

Extending inwardly of the wall extension 18 at its upper end is an annular plate 18.2, the latter plate extending inwardly for sliding contact with the piston rod 16.1. At its inner periphery, the annular plate 18.2 is provided with a peripheral groove carrying a lubricated packing gland 18.3 which establishes a substantially airtight seal between the annular plate and the piston rod. The upper surface 18.4 of the annular plate confronts the lower surface 16.9 of the piston plate, and these surfaces cooperate with the cylinder walls 14.2 and the exterior wall of the piston rod 16.1 to define a cavity 22 which, as will be subsequently explained, expands and contracts in volume as the cushion is depressed and as it returns toward its initial position. An air flow passage 18.5 is formed in the wall extension 18 and annular plate 18.2 to provide communication between the interior of the cavity 22 and the exterior of the cushion. A check valve 18.6 is provided at the exterior port of the passage 18.5 to permit air to enter the cavity but to prevent air from escaping therefrom through the passage 18.5.

The bottom plate 18 of the piston rod, together with the upright cylindrical walls 18 and 16.6 of the cylinder and piston and the lower surface 18.7 of the annular plate, provides an air chamber designated generally as 20. An air conduit 17.1 is formed in the bottom plate 17 of the piston and communicates the air chamber 20 with a source of air under substantially constant pressure such as a factory air line 17.2. A surge tank, (not shown) or other device, may be connected into the air line 17.2 so as to damp out and hence minimize pressure fluctuations in the air line and in the air chamber 20. A conduit 17.3 extends upwardly through the center of the piston rod 16.1, and communicates the air chamber 21 with the air chamber 20 to maintain a pressure equilibrium between the two chambers. The conduit 17.2 branches transversely at its lower end for connection into the air chamber 20, and is desirably of sufficient size as to readily pass air between the chambers 20 and 21.

It will be understood that under the force of a downwardly moving ram, the cylinder 14, its wall extension 18 and the annular plate 18.2 will move downwardly with respect to the piston, into the depressed position shown in FIG. 2. As the upper surface 18.4 of the annular plate recedes from the lower surface 16.9 of the piston plate, the cavity 22 bounded by these surfaces expands in volume, air entering the cavity through the check valve 18.6 and through the flow passage 18.5. At the same time, the cylinder and annular plate 18.2 push downwardly upon the air in the air chambers 20 and 21. Since the air in the chambers 20, 21 is under a regulated and desirably super-atmospheric pressure, the downward movement of the cylinder and its associated parts is retarded and cushioned. When the downward force of the ram is removed, the pressurized chambers 20, 21 act upwardly on the lower surfaces 18.7 and 14.4 of the annular plate and strike plate, respectively, causing the cylinder and its associated parts to move upwardly, the cushion thus returning to its original height. The cavity 22 concurrently decreases in volume, and the air within the cavity 22 is prevented from rushing outwardly through the airflow passage 18.5 by the check valve 18.6. The air within the cavity thus becomes increasingly pressurized. If the air within the cavity 22 were permitted to escape freely, the cushion would regain its original height rapidly, resulting in a jolt with the accompanying hammering effect, as described above. On the other hand, if air were prevented completely from escaping from the cavity 22, the cylinder would eventually move upwardly to an equilibrium position short of its original height as the downward force resulting from the pressurized air in the cavity 22 is balanced by the upward force generated by the air under regulated pressure in the air chambers 20 and 21.

To insure that the cushion returns completely to its original height following each pressing operation, and to progressively slow the upward velocity of the cylinder as the latter die approaches its original height, a decelerator valve, designated generally as 24 is provided to bleed air from the cavity 22 at a controlled rate. The decelerator valve communicates with the air in the air chambers 20 and 21, and also communicates with the air in the cavity 22 and is responsive to both pressures, the valve opening when the cavity pressure exceeds the 4 cushion pressure by a given ratio as will now be described.

Referring now to FlG. 4, the decelerator valve 24 is mounted within a recess 18.71 in the annular plate 18.2. At its inner end, the recess has an upwardly directed port 18.8 communicating the recess with the cavity 22. Spaced outwardly from the inward end of the recess is a port 18.9 which communicates the recess with the air in the air chamber 20. The decelerator valve 24 may have a generally cylindrical configuration, and has a central bore 24.1 communicating at one end with the port 18.8 and at its other end with the port 18.9. A piston 24.2 is carried slideably within the bore 24.1, and may have a groove about its periphery in which may be seated a packing gland 24.3 to render the seal between the piston and bore substantially airtight. At its outward end, the decelerator valve is provided with a cap 24.4 which seals, in an airtight fashion, the outer end of the bore. The valve 24 may be mounted in the recess 18.71 by means of a split ring 24.5 of a type known to the art, and the recess 18.71 is open to the exterior of the press.

The outer end 24.6 of the valve piston 24.2 contains a central spring recess 24.7. Across the valve bore 24.1 between the end 24.6 of the piston and the cap or seal 24.4 is placed a retainer ring 24.8, the latter having a spring guide rod 24.9 extending inwardly of the piston recess 24.7 with the end of the guide spaced from the inner end of the recess. A helical spring 25 is carried about the guide rod 24.9 and bears against the retainer 24.8 and the inner surface of the piston recess 24.7 to gently urge the piston inwardly.

The inner end of the valve bore 24.1 is provided with an inwardly directed annular flange 25.1 forming a valve seat, and between the inner end 25.2 of the piston and the valve seat is inserted a valve plug which may be in the form of a sphere 25.3. As the sphere is pressed inwardly by the piston, the sphere seats against the valve seat 25.1 to prevent air from flowing through the opening in the valve seat.

Formed longitudinally in the body of the decelerator valve 24 is a relief duct 25.4 which communicates at its inward end with the piston side of the valve seat opening. The other side of the valve seat opening communicates with the cavity passage 18.8, such that when the sphere 25.3 is moved outwardly (to the left in FIG. 4) away from seating engagement with the valve seat 25.1, air from the cavity may escape through the opening in the valve seat and through the relief duct 25.4 to the atmosphere.

Also formed longitudinally in the valve body is a bypass duct 25.5 which communicates at its inner and with the cavity passage 18.8. The outer end of the bypass duct is threaded to receive a flow control valve 25.6, which in its preferred embodiment is a needle valve communicating the interior of the bypass duct with the atmosphere through holes 25.7 in the exterior flow control valve surface. The needle of the needle valve includes an outwardly extending, exteriorly accessible valve stem 25.8 which may be slotted to receive a screwdriver and which may be turned to regulate the flow rate of air passing through the flow control valve.

As described above, the valve bore 24.1 communicates adjacent its exterior end with the air within the die cushion chamber 20, the exterior end 24.6 of the piston thus being subjected to the pressure of that chamber. The sphere 25.3, which normally seals the opening in the valve seat 25.1, is exposed to the pressure of air within the cavity 22, and it will be understood that the sphere valve will be forced out of its seat ing engagement with the valve seat when the force resulting from the air pressure within the cavity acting on the sphere is greater than the force acting on the exterior end 24.6 of the piston resulting from the pressure of the air within the chamber 20. The spring 25 gener ates only negligible force against the piston, this force being sufficient only to move the piston and sphere inwardly (to the right in FIG. 4) in the absence of other forces acting on the piston to normally retain the ball in its seating position in the valve seat. The magnitude of the forces generated upon the outward end 24.6 of the piston and upon the exposed surface of the sphere by the pressure of air in the chamber and cavity 22, re spectively, is dependent upon the effective hydraulic area of the piston end 24.6 and sphere 25.3 which are exposed to such pressures. The area at the outer end 24.6 of the piston against which the pressure of the air in the chamber 20 acts to move the piston inwardly (to the right in FIG. 4) is termed herein the effective hydraulic area of the piston end, and similarly, the effective area of the sphere against which the pressure of the air in the cavity acts to move the sphere out of its seating engagement with the valve seat is termed the effective hydraulic area of the sphere. If the effective hydraulic area of the sphere is substantially less than that of the piston, it will be understood that the cavity pres sure must be substantially greater than the cushion pressure in order for the valve to open.

Referring again to FIGS. 1-3, it will be noted that the regulated air pressure within the air chambers 20 and 21 of the cushion act upon the lower surfaces of both the strike plate 14.1 and the annular plate 18.2, the resultant upward force on the cylinder 14 being equal to the regulated air pressure multipled by the sum of the effective hydraulic areas of the lower surfaces of the strike plate and annular plate. Neglecting the weight of the cylinder, friction, and any work which the cylinder must do as it moves upwardly (such as removing a workpiece from the press), the total force which resists the upward movement of the cylinder is equal to the force generated by the increasing air pressure within the cavity 22 upon the effective hydraulic area of the upper surface 18.4 of the annular plate. In the embodi' ment depicted in the drawing, it is evident that the effective hydraulic area upon which the pressure of the air within the air chambers 20 and 21 acts is considerably greater than the effective hydraulic area upon which the pressure of the air in the cavity acts. It may be shown that the upward movement of the cylinder 14 would cease (that is, the upward and downward forces on the cylinder would balance) when the ratio of the air pressure of the cavity to that of the chambers 20, 21 is increased to the ratio of the sum of the hydraulic areas of the chambers 20 and 21 to the effective hydraulic area of the cavity.

To insure that the decelerator valve opens before the thus described balance occurs, the effective hydraulic areas of the valve piston and sphere are chosen so that the sphere disengages its valve seat when the above ratio of air pressures is slightly less than the ratio which balances forces on the cylinder. Since the air pressure in the cavity acts simultaneously on the effective by draulic areas of the cavity and the sphere, and since the air chamber pressure acts simultaneously upon the effective hydraulic areas of the chambers 20, 21 and the cylinder must balance and tvhamhl'r) lrlmmhl'r) mm trul'l Assume that simultaneously the sphere 25.3 is held by the gentle, negligable pressure of the spring 25 against the valve seat, other forces on the piston 24.2 being balanced. Then:

ts-ulum lrm') my-plum wnnmhen Combining (l) and (2), at equilibrium:

ns-Plum tmrl To avoid attaining this equilibrium, the value of A may be increased or the value of A may be decreased such that the ratio A ,/A is slightly greater than the ratio A lA In the above discussion, the weight of the cylinder 14 and any work done by the cylinder as it returns to its original height were neglected. Such items are of very small magnitude in comparison to the forces resulting from pressures within the cavity 22 and chambers 20 and 21. The above described non-equilibrium ratio of effective hydraulic areas assures that the cushion will return completely to its original height. The speed of return of the cushion to its original height may be very closely controlled by adjustment of the flow control valve 25.6.

In use, the die cushion is depressed by downward force of the ram, the air chambers 20, 21 decreasing in volume but maintaining a substantially constant air pressure through the use ofa surge tank or the like connected into the line 17.2. Simultaneously, the cavity 22 expands in volume, exterior air entering the cavity relatively freely through the air passage 18.5. As the ram is retracted, the cushion begins to regain its original height under the pressure of the air in the air chambers 20, 21. Air within the cavity is prevented from escaping, however, by the check valve 18.6, and the air pressure within the cavity increases, resulting in an increasing downward force resisting upward movement of the cylinder 14. When the ratio of cavity pressure to chamber pressure reaches a predetermined value (dependent upon the relationship of the various effective hydraulic areas as discussed above), the decelerator valve begins to open against the light spring pressure of the valve spring 25, permitting air from the cavity to escape outwardly to the atmosphere and assuring continued upward movement of the cylinder 14. The cylinder is thus continuously decelerated at a substantially constant rate in its upward movement, and comes to rest gently at its initial height. The speed with which the cylinder regains its initial height may be finely adjusted by means of the flow control valve 25.6.

Thus, manifestly, we have provided a pneumatic cushion for a power press which is particularly suited for use in high speed presses and which is automatically and smoothly decelerated in its return following a 7 pressing operation so as to come to rest gently at its initial height, thus avoiding jolting and hammering associated with previous cushions.

While we have described a preferred embodiment of the present invention, it should be understood that various changes, adaptations, and modifications maybe made therein without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is I l. A controlled return pneumatic ,die cushion for a power press, the cushion having an air chamber under substantially constant pressure which respectively contracts and expands in volume as the cushion is decreased in height under the force of a power press ram and as the cushion recovers its original height when the force of the ram is removed, the cushion including a cavity which respectively expands and contracts in volume as the height of the cushion decreases and increases, the cushion including a decelerator valve for bleeding air in a controlled manner from the cavity to control the rate at which the cushion returns to its original height, the valve communicating with the air chamber and the cavity and responsive to the ratio of cavity pressure to air chamber pressure to open when a given ratio has been exceeded 2. The die cushion of claim 1 wherein the decelerator valve includes a plug, a conduit communicating one side of the plug with air pressure in the cavity, another conduit communicating the other side of the plug with air pressure in the air chamber, and a valve seat engageable with the one side of the plug to close the valve.

3. The die cushion of claim 2 in which the one and the other of the plug sides have effective hydraulic areas upon which the cavity pressure and air chamber pressure act, respectively, to move the plug into and out of engagement with the valve seat, and wherein the cavity and air chamber each have effective hydraulic areas upon which respectively act the cavity and chamber air pressures in opposition to one another, the ratio of the effective hydraulic areas of the one side of the plug to the other side being slightly greater than the ratio of the effective hydraulic area of the cavity to that of the air chamber. v

4. The pneumatic die cushion of claim 1 in which the decelerator valve includes a manually operable flow control valve communicating with the cavity for bleeding air therefrom to the atmosphere, whereby the rate of return of the cushion to its initial height may be closely regulated.

5. A controlled return pneumatic die cushion for a power press, the cushion including an air chamber under substantially constant pressure which respectively contracts and expands in volume as the cushion is decreased in height under the force'of a power press ram and as the cushion recovers its original height when the force ofthe ram is removed, the air chamber having an effective hydraulic area upon which thepres sure therein acts to urge the cushion to its original height, the cushion including a cavity which respectively expands and contracts in volume as the height of the cushion decreases and increases the cavity having an effective hydraulic area upon which the pressure therein acts to oppose return movement of the cushion, and a decelerator valve for bleeding air in a controlled manner from the cavity to control the rate at which the cushion returns to its original height, the valve including a valve seat, a plug having one side engageable with the valve seat and in communication with pressure in the cavity and an opposed side in communication with pressure in the air chamber, the sides of the plug having effective hydraulic areas upon which act the cavity and air chamber pressures, respectively, to move the plug out of and into engagement with the valve seat, the ratio of the effective hydraulic area of the one plug side to that of the other plug side being slightly greater than the ratio of the effective hydraulic area of the cavity to that of the air chamber, whereby the plug disengages the valve seat to enable air to escape from the cavity when a given ratio of cavity pressure to cushion pressure has been exceeded, thereby decelerating the return movement of the cushion to its original height.

6. The die cushion of claim 5 wherein the effective hydraulic area of the air chamber is approximately twice that of the cavity.

7. The die cushion of claim 5 wherein the decelerator valve includes a manually operable flow-control valve communicating with the cavity for continuously bleeding air from the cavity as the cushion returns to its initial height.

8. A controlled return, pneumatic die cushion for a power press, the cushion comprising an upright cylinder and piston having confronting surfaces defining the top and bottom walls of an air chamber, the cylinder being movable downwardly upon the piston under the force of a power press ram to reduce the volume of the air chamber, the piston and cylinder further having opposed, confronting surfaces defining the upper and lower walls of a cavity which expands in volume as the cylinder moves downwardly upon the piston and which retracts as the cylinder regains its initial heighth when the pressure of the ram of a press is removed, a source of air under substantially constant pressure communicating with the air chamber, and a decelerator valve carried by the cylinder and including a plug having one side in flow communication with the cavity and an opposed side in flow communication with the air chamber, the valve including a valve seat engageable by the one side of the plug, the opposed sides of the plug having effective hydraulic areas upon which act the pressures in the cavity and air chamber, respectively, to urge the plug out of and into engagement with the valve seat, the ratio of the effective hydraulic area of the one side of the plug to the opposed side thereof being slightly greater than the ratio of the effective hydraulic area of the cavity to that of the air chamber.

9. The die cushion of claim 8 wherein the decelerator valve includes a spring gently urging the plug into engagement with valve seat, and an externally operable flow control valve communicating with the cavity for bleeding air continuously from the cavity to the atmosphere.

10. The die cushion of claim 8 wherein the piston includes an upright, stationary piston rod, a piston head at the upper end of the rod, and wherein the cylinder includes an upper strike plate, confronting surfaces of the strike plate and piston head defining the upper and lower walls of the air chamber, the piston rod and head having an air flow passage therein communicating the air chamber with an external source of air under substantially constant pressure, the cylinder including an annular plate spaced below the strike plate and extending inwardly into sliding contact with the piston rod below the piston head, confronting surfaces of the piston head and annular plate defining upper and lower walls of the cavity, thepiston rod having at its lower end an outwardly extending baseplate with upright peripheral walls slidingly engaging the walls of the cylinder, confronting surfaces of the baseplate and annular plate respectively defining lower and upper walls of a second air chamber in flow communication with the first air chamber, the lower surfaces of the cylinder strike plate and annular plate each contributing to the effective hydraulic area against which the substantially constant air pressure acts to urge the cylinder upwardly.

11. A controlled return pneumatic die cushion for a power press, the cushion having an air chamber under substantially constant pressure for returning the die cushion upwardly to its original height following a press operation, the air chamber respectively contracting and expanding in volume as the cushion is decreased in height under the force of a power press ram and as the cushion recovers its original height when the force of the ram is removed, the cushion including a cavity which respectively expands and contracts in volume as the height of the cushion decreases and increases, air pressure within the cushion providing a force in opposition to the force provided by air pressure in the air chamber as the die cushion is returned upwardly to its original height, and a decelerator valve for bleeding air in a controlled manner from the cavity to control the rate at which the cushion returns to its original height, the valve communicating with the air chamber and the cavity and responsive substantially only to the ratio of cavity pressure to air chamber pressure to open when a given ratio of said pressures has been exceeded, said given ratio being such as to cause the valve to bleed air from the cavity so that the force resulting from the cavity pressure does not rise to balance the force resulting from the air chamber pressure, whereby the die cushion is decelerated in its upward movement to its original height. 

1. A controlled return pneumatic die cushion for a power press, the cushion having an air chamber under substantially constant pressure which respectively contracts and expands in volume as the cushion is decreased in height under the force of a power press ram and as the cushion recovers its original height when the force of the ram is removed, the cushion including a cavity which respectively expands and contracts in volume as the height of the cushion decreases and increases, the cushion including a decelerator valve for bleeding air in a controlled manner from the cavity to control the rate at which the cushion returns to its original height, the valve communicating with the air chamber and the cavity and responsive to the ratio of cavity pressure to air chamber pressure to open when a given ratio has been exceeded.
 2. The die cushion of claim 1 wherein the decelerator valve includes a plug, a conduit communicating one side of the plug with air pressure in the cavity, another conduit communicating the other side of the plug with air pressure in the air chamber, and a valve seat engageable with the one side of the plug to close the valve.
 3. The die cushion of claim 2 in which the one and the other of the plug sides have effective hydraulic areas upon which the cavity pressure and air chamber pressure act, respectively, to move the plug into and out of engagement with the valve seat, and wherein the cavity and air chamber each have effective hydraulic areas Upon which respectively act the cavity and chamber air pressures in opposition to one another, the ratio of the effective hydraulic areas of the one side of the plug to the other side being slightly greater than the ratio of the effective hydraulic area of the cavity to that of the air chamber.
 4. The pneumatic die cushion of claim 1 in which the decelerator valve includes a manually operable flow control valve communicating with the cavity for bleeding air therefrom to the atmosphere, whereby the rate of return of the cushion to its initial height may be closely regulated.
 5. A controlled return pneumatic die cushion for a power press, the cushion including an air chamber under substantially constant pressure which respectively contracts and expands in volume as the cushion is decreased in height under the force of a power press ram and as the cushion recovers its original height when the force of the ram is removed, the air chamber having an effective hydraulic area upon which the pressure therein acts to urge the cushion to its original height, the cushion including a cavity which respectively expands and contracts in volume as the height of the cushion decreases and increases, the cavity having an effective hydraulic area upon which the pressure therein acts to oppose return movement of the cushion, and a decelerator valve for bleeding air in a controlled manner from the cavity to control the rate at which the cushion returns to its original height, the valve including a valve seat, a plug having one side engageable with the valve seat and in communication with pressure in the cavity and an opposed side in communication with pressure in the air chamber, the sides of the plug having effective hydraulic areas upon which act the cavity and air chamber pressures, respectively, to move the plug out of and into engagement with the valve seat, the ratio of the effective hydraulic area of the one plug side to that of the other plug side being slightly greater than the ratio of the effective hydraulic area of the cavity to that of the air chamber, whereby the plug disengages the valve seat to enable air to escape from the cavity when a given ratio of cavity pressure to cushion pressure has been exceeded, thereby decelerating the return movement of the cushion to its original height.
 6. The die cushion of claim 5 wherein the effective hydraulic area of the air chamber is approximately twice that of the cavity.
 7. The die cushion of claim 5 wherein the decelerator valve includes a manually operable flow-control valve communicating with the cavity for continuously bleeding air from the cavity as the cushion returns to its initial height.
 8. A controlled return, pneumatic die cushion for a power press, the cushion comprising an upright cylinder and piston having confronting surfaces defining the top and bottom walls of an air chamber, the cylinder being movable downwardly upon the piston under the force of a power press ram to reduce the volume of the air chamber, the piston and cylinder further having opposed, confronting surfaces defining the upper and lower walls of a cavity which expands in volume as the cylinder moves downwardly upon the piston and which retracts as the cylinder regains its initial heighth when the pressure of the ram of a press is removed, a source of air under substantially constant pressure communicating with the air chamber, and a decelerator valve carried by the cylinder and including a plug having one side in flow communication with the cavity and an opposed side in flow communication with the air chamber, the valve including a valve seat engageable by the one side of the plug, the opposed sides of the plug having effective hydraulic areas upon which act the pressures in the cavity and air chamber, respectively, to urge the plug out of and into engagement with the valve seat, the ratio of the effective hydraulic area of the one side of the plug to the opposed side thereof being slightly greater than the ratio of the effective hydraulic area of the Cavity to that of the air chamber.
 9. The die cushion of claim 8 wherein the decelerator valve includes a spring gently urging the plug into engagement with valve seat, and an externally operable flow control valve communicating with the cavity for bleeding air continuously from the cavity to the atmosphere.
 10. The die cushion of claim 8 wherein the piston includes an upright, stationary piston rod, a piston head at the upper end of the rod, and wherein the cylinder includes an upper strike plate, confronting surfaces of the strike plate and piston head defining the upper and lower walls of the air chamber, the piston rod and head having an air flow passage therein communicating the air chamber with an external source of air under substantially constant pressure, the cylinder including an annular plate spaced below the strike plate and extending inwardly into sliding contact with the piston rod below the piston head, confronting surfaces of the piston head and annular plate defining upper and lower walls of the cavity, the piston rod having at its lower end an outwardly extending baseplate with upright peripheral walls slidingly engaging the walls of the cylinder, confronting surfaces of the baseplate and annular plate respectively defining lower and upper walls of a second air chamber in flow communication with the first air chamber, the lower surfaces of the cylinder strike plate and annular plate each contributing to the effective hydraulic area against which the substantially constant air pressure acts to urge the cylinder upwardly.
 11. A controlled return pneumatic die cushion for a power press, the cushion having an air chamber under substantially constant pressure for returning the die cushion upwardly to its original height following a press operation, the air chamber respectively contracting and expanding in volume as the cushion is decreased in height under the force of a power press ram and as the cushion recovers its original height when the force of the ram is removed, the cushion including a cavity which respectively expands and contracts in volume as the height of the cushion decreases and increases, air pressure within the cushion providing a force in opposition to the force provided by air pressure in the air chamber as the die cushion is returned upwardly to its original height, and a decelerator valve for bleeding air in a controlled manner from the cavity to control the rate at which the cushion returns to its original height, the valve communicating with the air chamber and the cavity and responsive substantially only to the ratio of cavity pressure to air chamber pressure to open when a given ratio of said pressures has been exceeded, said given ratio being such as to cause the valve to bleed air from the cavity so that the force resulting from the cavity pressure does not rise to balance the force resulting from the air chamber pressure, whereby the die cushion is decelerated in its upward movement to its original height. 